Parasitic helminth infections in animals, including humans, are typically treated by chemical drugs, because there are essentially no efficacious vaccines available. One disadvantage with chemical drugs is that they must be administered often. For example, dogs susceptible to heartworm are typically treated monthly to maintain protective drug levels. Repeated administration of drugs to treat parasitic helminth infections, however, often leads to the development of resistant helminth strains that no longer respond to treatment. Furthermore, many of the chemical drugs cause harmful side effects in the animals being treated, and as larger doses become required due to the build up of resistance, the side effects become even greater. Moreover, a number of drugs only treat symptoms of a parasitic disease but are unable to prevent infection by the parasitic helminth.
It is particularly difficult to develop vaccines against parasitic helminth infections both because of the complexity of the parasite's life cycle and because, while administration of parasites or parasite antigens can lead to the production of a significant antibody response, the immune response is typically not sufficient to protect the animal against infection.
As an example of the complexity of parasitic helminths, the life cycle of D. immitis, the helminth that causes heartworm, includes a variety of life forms, each of which presents different targets, and challenges, for immunization. Adult forms of the parasite are quite large and preferentially inhabit the heart and pulmonary arteries of an animal. Sexually mature adults, after mating, produce microfilariae which traverse capillary beds and circulate in the vascular system of the dog. One method of demonstrating infection in the dog is to detect the circulating microfilariae.
If the dog is maintained in an insect-free environment, the life cycle of the parasite cannot progress. However, when microfilariae are ingested by the female mosquito during blood feeding on an infected dog, subsequent development of the microfilariae into larvae occurs in the mosquito. The microfilariae go through two larval stages (L1 and L2) and finally become mature third stage larvae (L3) which can then be transmitted back to the dog through the bite of the mosquito. It is this L3 stage, therefore, that accounts for the initial infection. As early as three days after infection, the L3 molt to the fourth larval (L4) stage, and subsequently to the fifth stage, or immature adults. The immature adults migrate to the heart and pulmonary arteries, where they mature and reproduce, thus producing the microfilariae in the blood. "Occult" infection with heartworm in dogs is defined as that wherein no microfilariae can be detected, but the existence of the adult heartworms can be determined through thoracic examination.
Heartworm not only is a major problem in dogs, which typically cannot even develop immunity upon infection (i.e., dogs can become reinfected even after being cured by chemotherapy), but is also becoming increasingly widespread in other companion animals, such as cats and ferrets. Heartworm infections have also been reported in humans. Other parasitic helminthic infections are also widespread, and all require better treatment, including a preventative vaccine program. O. volvulus, for example, causes onchocerciasis (also known as river blindness) in humans. Up to 50 million people throughout the world are reported to be infected with O. volvulus, with over a million being blinded due to infection. Brugia filariids can infect humans and other animals, causing diseases including filariasis (including lymphatic filariasis), elephantiasis and tropical eosinophilia.
Although many investigators have tried to develop vaccines based on specific antigens, it is well understood that the ability of an antigen to stimulate antibody production does not necessarily correlate with the ability of the antigen to stimulate an immune response capable of protecting an animal from infection, particularly in the case of parasitic helminths. A large number of materials are immunogenic and produce sera which test positive in immunoassays for ability to react with the immunizing antigen, but which fail to protect the hosts against infection. Accordingly, the use of serum simply resulting from immunization or from infection by a parasitic helminth to screen for candidate vaccines does not provide sufficient specificity to identify protective immunogens. On the other hand, serum or other components of blood from immunized animals which is demonstrably protective against infection would contain antibodies, cells, or other factors that could selectively bind to potential antigens that, if used as therapeutic compositions, would elicit immune responses that protect against challenge. A method to use serum from immune animals to identify candidate parasitic helminth vaccines is disclosed in U.S. patent application Ser. No. 08/101,283, (abandoned) ibid., also published as PCT International Publication No. WO 92/13560, by Grieve et al., on Aug. 20, 1992.
An alternative approach to finding a suitable parasitic helminth vaccine has been to attempt to identify prominent antigens in the infective stage of the helminth. Researchers have identified several proteins in the infective stage of D. immitis., including, for example, a 35-kilodalton (kD) major surface antigen of D. immitis third stage larvae (Philipp, et al., 1986, J. Immunol. 136, 2621-2627; Ibrahim, et al., 1989, Parasitol. 99, 89-97; Scott, et al, 1990, Acta Tropica 47, 339-353) as well as three major surface proteins of the L4 having molecular weights of 150 kD, 52 kD, and 25 kD (Davis, et al., 1988, Abstract 404, 37th Annual Meeting, Am. Soc. Trop. Med. Hyg.). Scott et al., ibid., also identified a number of other proteins on the surface of D. immitis having molecular weights ranging from 3 kD to 66 kD. None of these proteins has yet been shown to be an effective vaccine.
Furthermore, although several Onchocerca genes have been isolated, genes encoding antigens targeted specifically to L3 and L4 stage larvae have apparently not been reported. In particular, genes encoding antigens that selectively bind to serum obtained from a host that is immune to Onchocerca infection (e.g., O. volvulus infection), apparently have not been isolated, nor apparently have such antigens been characterized.
As such, there remains a need to identify an efficacious composition that protects animals against diseases caused by parasitic helminths and that, preferably, also protects animals from infection by such helminths.